Signs incorporating visual characters such as numerals, letters, words, phrases, sentences or other indicia are utilised in offshore aquatic environments such as subsea locations, for marking or providing information about an object located below the surface or on the seabed. Such signs have particular utility in the oil and gas exploration and production industry. Aquatic growth, such as barnacles, algae and other aquatic vegetation is a problem in these environments, and can cause fouling of the signs so that they become illegible.
In an effort to overcome these problems, polymeric anti-fouling silicone-based compositions were developed in the 1980s which resist such aquatic growth, as disclosed for example in European Patent Publication No. 0171110. Signs manufactured from or incorporating these materials comprise a semi-compatible lubricant, which has a tendency to leach out from the surface of the sign in use, thereby preventing aquatic growth and fouling of the sign.
International Patent Publication No. WO 2005/042220 discloses a moulding method and apparatus used in the manufacture of anti-fouling signs. In general terms, the signs are manufactured by constructing a mould; forming a character template on a surface of the mould; pouring a silicone material into the mould to form visual characters defined by the template; partially curing the characters; removing the template; coupling a backing sheet to the mould; inclining the mould from the horizontal; pouring a relief, substrate silicone material (of a different colour) into the mould through a small aperture; curing the sign; and then releasing the completed sign from the mould. Whilst signs manufactured according to this method function well in terms of resisting aquatic growth, there are various disadvantages or problems associated with the moulding method and the resultant signs.
In particular, the nature of the materials used to manufacture the signs creates difficulties in bonding the signs to a subsea object. To address this, the signs incorporate a backing layer with an irregular surface. A preferred sheet is of a type including loops, such as those provided on hook-and-loop fastener sheets, sold under the VELCRO® Trade Mark. During manufacture of the sign, the silicone material forms a mechanical connection with the loops on the backing layer. The completed sign may then be attached to an object to be located subsea using a suitable adhesive which bonds to the backing layer. However, a thin layer of a plastic material is provided on the backing sheet on the surface opposite that bearing the loops, to prevent uncured silicone from seeping through the VELCRO® sheet during manufacture.
This thin layer of plastics material can be easily detached from the backing sheet, reducing mechanical strength of the sign, in use. Furthermore, the backing layer adds to the manufacturing time and cost of the sign, and reduces flexibility.
Employing an adhesive to mount polymeric antifouling markers can provide satisfactory results if carried out correctly. However, the success of the bond depends upon the skill and knowledge of the individual carrying out the process. If the methods are not carried out correctly then a weaker than expected bond forms between the marker and the subsea structure which can result in the marker becoming detached from the subsea structure such that the associated information contained on the marker is lost. Adhesives for subsea use are often formed by mixing two components, for example a two-part polyurethane-based adhesives. The adhesive bonding system commercially available from Champion Environmental Techologies under the trade mark AQUAHESIVE™ is an example of such a system, and has an accepted operational lifetime subsea of around 50 years.
With two-part adhesive systems incorrect mixing can compromise the effectiveness of the adhesive. Even if correctly mixed, the adhesive has a limited useful period during which it can be applied, which may not be more than thirty or forty minutes. This means that individuals may attempt to use the subsea adhesives at the end of their useful lifetimes, which results in poor adhesion of the subsea markers. The adhesive needs to be carefully applied, with incorrect application resulting in a deficient bond. Furthermore, if the adhesion process is carried out on land then a number of factors have been found to weaken the bond, e.g. heat and humidity.
These adhesion issues are exacerbated due to the harsh conditions in which the markers operate. Accessing the subsea structures, e.g. for intervention or maintenance purposes, typically requires the use of divers and/or remotely operated vehicles (ROV). It is common for the diver or the ROV to come into contact with the marker when trying to read the information carried by the marker. This can act to damage the marker or completely dislodge the marker if not properly bonded with the subsea structure. In practice, it is found that if the edge of a polymeric antifouling marker is damaged then this acts as a point of weakness within the marker which can result in further tearing or disintegration of the marker.
The subsea structures and markers are also required to be cleaned during their operating lifetimes. Typically this is achieved by the employment of high pressure (5000 psi) water jet cleaning systems. A marker can be damaged by such a process or, if incorrectly bonded, the marker can be dislodged as a result of the jet washing process.
An additional problem with many of the prior art mounting arrangements is that corrosion of the subsea structure may be caused or accelerated by a volume of static or stagnant salt water between the subsea structure and the subsea antifouling sign.
One approach to mounting polymeric antifouling signs is to manufacture them in the form of a tag wherein apertures are formed in the backing substrate of the sign (FIG. 1). This design allows the sign 10 to be attached to a subsea structure 11 by the employment of a steel wire 12, or other similar attachment means, which is threaded through the apertures 13 without the use of adhesives. This method simplifies the attachment process so that less skilled individuals can attach the signs prior to deployment subsea. However, signs attached in this manner are found to move about the mounting such that when required to be detected at a later date, the information can be hidden from a diver or ROV attempting to locate the sign.
Mountings of the type in FIG. 1 have been configured to provide stand-off between the sign and the structure to which it is mounted. This reduces the effects of stagnant water behind the sign, but does not eliminate the problem and resulting corrosion remains a concern.
Another method of using polymeric antifouling signs is described in UK patent publication number GB 2473526. The method provides a backing member and a fixing frame which attaches to the backing member to secure the perimeter of the antifouling marker between the fixing frame and the backing member. Whilst this method is convenient to protect the antifouling marker and the identification tag from physical damage, it has other drawbacks such as involving multiple fabrication and assembly steps, the need of skilled labour to weld the fixing frame to the backing member, and the additional cost of the mount. In addition, the system of GB 2473526 uses bolts. There is a perception that the bolts may become loose over time and potentially damage paints or coatings on the structure. There may also be concerns about the long term effects caused by the inside of bolt holes not being protected by corrosion resistant coatings or paint schemes.
GB 2434022 describes an alternative mounting method for subsea antifouling signs wherein a silicone-based adhesive compatible with the material of the subsea antifouling sign is used.
It is an object of an aspect of the present invention to obviate or at least mitigate the foregoing disadvantages of previous methods of mounting a subsea antifouling sign to a subsea structure. Another object of the invention is to provide an easy, reliable and fast method of mounting a subsea antifouling sign to a subsea structure. A further object of the invention is to provide a subsea antifouling sign mounted on a subsea structure which is securely attached to the subsea structure for extended periods of time in the harsh conditions encountered in subsea environments. Further aims of the invention will become apparent from the following description.